Hybrid vehicle control when towing

ABSTRACT

A method for controlling a hybrid vehicle having a trailer hitch includes detecting the trailer hitch being in use, commanding an engine to stop, and commanding the engine to start in response to a torque to accelerate being within a predetermined offset below an unladen vehicle torque pull up schedule.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. application Ser. No. 14/052,993filed Oct. 14, 2013, the disclosure of which is hereby incorporated inits entirety by reference herein.

TECHNICAL FIELD

Various embodiments relate to a hybrid vehicle and control of thevehicle while the vehicle is towing.

BACKGROUND

Hybrid vehicles may have first and second prime movers, such as aninternal combustion engine, and an electric machine. The vehicle mayoperate using either prime mover, or using both simultaneously. Thevehicle may operate electrically depending on the amount of availableelectrical power and the vehicle loads, including driver demand andother vehicle accessory loads. When the battery state of charge reachesa lower limit, the engine may be started to provide additional power tothe vehicle. The engine may also be started when the power or torquerequirement of the vehicle will exceed that available from the electricmachine.

The hybrid vehicle may be equipped with a towing package. A towingpackage includes a trailer hitch and a trailer light electricalconnection. When a load, such as a trailer, is connected to the trailerhitch of the hybrid vehicle, the added load on the vehicle caused bypulling the trailer may exceed the electrical capability of the electricmachine, and require the engine to be operating. A delay associated withstarting or restarting the engine, or pulling up the engine, duringvehicle operation may cause driveline disturbances, or may not meetvehicle drivability expectations.

SUMMARY

According to an embodiment, a method for controlling a hybrid vehiclehaving an engine is provided. The engine is automatically stopped inresponse to vehicle power demand dropping below currently availableelectrical power while trailering. The engine is automatically startedwhen vehicle power demand is at a first offset below currently availableelectrical power to reduce a time delay for starting the engine andincrease a currently available vehicle power to move the vehicle andtrailer. The first offset is greater than a second offset for anunloaded vehicle.

According to another embodiment, a hybrid vehicle is provided with anengine, an electric machine, a trailer hitch, and a controller. Thecontroller is configured to, in response to the vehicle trailering,command the engine to start at a torque offset below an engine torquepull up schedule for an unladen vehicle, the torque offset based onvehicle loading.

According to yet another embodiment, a method for controlling a hybridvehicle having a trailer hitch is provided. The method detects thetrailer hitch being in use. An engine is commanded to stop to increasefuel efficiency. In response to a torque to accelerate the vehicle beingwithin a predetermined offset below an engine torque pull up schedulefor an unladen vehicle, the engine is commanded to start to reduce atime delay for starting the engine and increase vehicle torque to movethe vehicle.

Various embodiments have associated, non-limiting advantages. Forexample, to increase fuel efficiency and user expectation of a hybridvehicle, the vehicle may be operated in an electric only mode with theelectric machine propelling the vehicle and the engine off even when thevehicle is towing. When the vehicle is towing, especially for a heavierload such as a 5000 pound, 10,000 pound, or heavier trailer, theelectric machine may have a reduced ability to accelerate and propel thevehicle and trailer due to the added weight of the trailer. Instead ofdisabling an electric only mode, as has been done in the prior art, thepresent disclosure allows the vehicle to operate electrically dependingon the vehicle demand. The present disclosure provides a vehicle wherethe engine may be shut down while the vehicle is operating and vehicledemand may be met electrically in order to conserve fuel and meet userexpectations. The engine may be pulled up in advance of a typicalvehicle pull up schedule in order to anticipate and meet user demand andvehicle demand with the added weight of a trailer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a hybrid vehicle according to anembodiment;

FIG. 2 is a schematic of the hybrid vehicle of FIG. 1;

FIG. 3 is a flow chart illustrating a method of controlling a hybridvehicle according to an embodiment;

FIG. 4 is a timing chart for the hybrid vehicle according to anembodiment;

FIG. 5 is a chart illustrating engine pull up based on vehicle power forthe hybrid vehicle according to an embodiment; and

FIG. 6 is a flow chart illustrating a method of determining a loadconnected to a trailer hitch of a vehicle according to an embodiment.

DETAILED DESCRIPTION

As required, detailed embodiments of the present invention are disclosedherein; however, it is to be understood that the disclosed embodimentsare merely exemplary of the invention that may be embodied in variousand alternative forms. The figures are not necessarily to scale; somefeatures may be exaggerated or minimized to show details of particularcomponents. Therefore, specific structural and functional detailsdisclosed herein are not to be interpreted as limiting, but merely as arepresentative basis for teaching one skilled in the art to variouslyemploy the present invention.

FIG. 1 illustrates a hybrid vehicle 10 according to an embodiment. Thevehicle 10 is illustrated as a pick-up truck; however, other vehiclestyles are also contemplated for use with the present disclosure. Thevehicle 10 has a first prime mover 12, and a second prime mover 14. Thefirst prime mover 12 may be an internal combustion engine. The secondprime mover 14 may be an electric machine, such as a motor/generator.The first prime mover 12 and/or the second prime mover 14 may be used topropel the vehicle.

The vehicle 10 has a towing package 16. The towing package 16 has a towbar 18. The tow bar 18 may include a hitch receiver (as shown), wherethe receiver may receive various ball mounts, and various ball sizes.Alternatively, the tow bar 18 may be a fixed drawbar with an integratedball mount. The tow bar 18 is connected to the vehicle chassis (notshown). The tow bar 18 may be one designed for use with various classesof hitches, such as Class I, Class II, Class III, or Class IV, or othersas are known in the art. The ball connected to the ball mount may beaccording to various standard ball sizes, such as a two inch ball, a 50mm ball, or others as are known in the art. Alternatively, the hitchcould receive other loads for transport, such as a bicycle rack, astorage box, a scooter lift, and the like.

The towing package 16 has an electrical connector 20. The electricalconnector 20 provides a signal to operate the trailer lights, includingbrake lights, turn signal lights, emergency lights, or the like. Thetowing package 16 may also have a trailer brake connector 22, such thatwhen a trailer equipped with brakes is connected, the trailer brakes maybe remotely controlled. The vehicle 10 may also be equipped with a loadleveling system 24, where the suspension and weight distribution betweenaxles may be adjusted based on the tongue weight of a trailer connectedto the towing package 16.

FIG. 2 illustrates a schematic diagram of a hybrid vehicle 30 accordingto an embodiment. The vehicle 30 may be used with a towing package 16 asshown in FIG. 1. The vehicle 30 includes an engine 32, and an electricmachine, which, in the embodiment shown in FIG. 2, is a motor generator(M/G) 34, and alternatively may be a traction motor. The M/G 34 isconfigured to transfer torque to the engine 32 or to the vehicle wheels36.

The M/G 34 is connected to the engine 32 using a first clutch 38, alsoknown as a disconnect clutch or the upstream clutch. A second clutch 40,also known as a launch clutch or the downstream clutch, connects the M/G34 to a transmission 42, and all of the input torque to the transmission42 flows through the launch clutch 40. Although the clutches 38, 40 aredescribed and illustrated as hydraulic clutches, other types ofclutches, such as electromechanical clutches may also be used.Alternatively, the clutch 40 may be replaced with a torque converterhaving a bypass clutch, as described further below. In differentembodiments, the downstream clutch 40 refers to various coupling devicesfor the vehicle 30 including a traditional clutch, and a torqueconverter having a bypass (lock-out) clutch. This configuration may usean otherwise conventional automatic step-ratio transmission with atorque converter and is sometimes referred to as a modular hybridtransmission configuration.

The engine 32 output shaft is connected to the disconnect clutch 38,which in turn is connected to the input shaft for the M/G 34. The M/G 34output shaft is connected to the launch clutch 40, which in turn isconnected to the transmission 42. The various components of the vehicle30 are positioned sequentially in series with one another. The launchclutch 40 connects the vehicle prime movers to the driveline 44, whichincludes the transmission 42, differential 46, and vehicle wheels 36,and their interconnecting components.

In another embodiment of the vehicle 30, the downstream clutch 40 is abypass clutch with a torque converter. The input from the M/G 34 is theimpeller side of the torque converter, and the output from the torqueconverter to the transmission 42 is the turbine side. The torqueconverter 40 transfers torque using its fluid coupling, and torquemultiplication may occur depending on the amount of slip between theimpeller and turbine sides. The bypass or lock-up clutch for the torqueconverter may be selectively engaged to create a mechanical orfrictional connection between the impeller side and the turbine side fordirect torque transfer. The bypass clutch may be slipped and/or openedto control the amount of torque transferred through the torqueconverter. The torque converter may also include a mechanical lockupclutch.

In the representative embodiment illustrated, the engine 32 is a directinjection engine. Alternatively, the engine 32 may be another type ofengine or prime mover, such as a port injection engine or fuel cell, oruse various fuel sources, such as diesel, biofuel, natural gas,hydrogen, or the like. In some embodiments, the vehicle 30 also includesa starter motor 48 operatively connected to the engine 32, for example,through a belt or gear drive. The starter motor 48 may be used toprovide torque to start the engine 32 without the addition of torquefrom the M/G 34, such as for a cold start, some high speed startingevents, or engine starts under towing load.

The M/G 34 is in communication with a battery 50. The battery 50 may bea high voltage battery. The M/G 34 may be configured to charge thebattery 50 in a regeneration mode, for example when vehicle power outputexceeds driver demand, through regenerative braking, or the like. TheM/G 34 may also be placed in a generator configuration to moderate theamount of engine 32 torque provided to the driveline 44. In one examplethe battery 50 is configured to connect to an external electric grid,such as for a plug-in hybrid electric vehicle (PHEV) with the capabilityto recharge the battery from an electric power grid, which suppliesenergy to an electrical outlet at a charging station. A low voltagebattery may also be present to provide power to the starter motor orother vehicle components, or low voltage power may be provided through aDC to DC converter connected to the battery 50.

In some embodiments, the transmission 42 is an automatic transmissionand connected to the drive wheels 36 in a conventional manner, and mayinclude a differential 46. The transmission 42 may be a step ratiotransmission. In other embodiments, the vehicle may have othertransmissions, including, continuously variable transmissions, manualtransmissions, and the like. The vehicle 30 is also provided with a pairof non-driven wheels, however, in alternative embodiments, a transfercase and a second differential can be utilized to positively drive allof the vehicle wheels.

The M/G 34 and the clutches 38, 40 may be located within a motorgenerator case 52, which may be incorporated into the transmission 42case, or alternatively, is a separate case within the vehicle 30. Thetransmission 42 has a gear box to provide various gearing ratios for thevehicle 30. The transmission 42 gearbox may include clutches andplanetary gearsets, or other arrangements of clutches and gear trains asare known in the art. In alternative embodiments, the transmission 42 isa continuously variable transmission or automated mechanicaltransmission. The transmission 42 may be an automatic six speedtransmission, other speed automatic transmission, or other gearbox as isknown in the art.

The transmission 42 is controlled using a transmission control unit(TCU) 54 or the like to operate on a shift schedule, such as aproduction shift schedule, that connects and disconnects elements withinthe gear box to control the gear ratio between the transmission outputand transmission input. The TCU 54 also acts to control the M/G 34, theclutches 38, 40, and any other components within the motor generatorcase 52.

An engine control unit (ECU) 56 is configured to control the operationof the engine 32. A vehicle system controller (VSC) 58 transfers databetween the TCU 54 and ECU 56 and is also in communication with variousvehicle sensors. The control system 60 for the vehicle 30 may includeany number of controllers, and may be integrated into a singlecontroller, or have various modules. Some or all of the controllers maybe connected by a controller area network (CAN) or other system. Thecontrol system 60 may be configured to control operation of the variouscomponents of the transmission 42, the motor generator assembly 52, thestarter motor 48 and the engine 32 under any of a number of differentconditions, including in a way that minimizes a delay in power deliveredby the vehicle in response to a user request when the vehicle is towingdue to a time associated with an engine pull up sequence.

It is recognized that any circuit or other electrical device disclosedherein may include any number of microprocessors, integrated circuits,memory devices (e.g., FLASH, random access memory (RAM), read onlymemory (ROM), electrically programmable read only memory (EPROM),electrically erasable programmable read only memory (EEPROM), or othersuitable variants thereof) and software which co-act with one another toperform operation(s) disclosed herein. In addition, any one or more ofthe electrical devices as disclosed herein may be configured to executea computer-program that is embodied in a non-transitory computerreadable medium that is programmed to perform any number of thefunctions as disclosed herein.

Under normal powertrain conditions (no subsystems/components faulted),the VSC 58 interprets the driver's demands (e.g. PRND and accelerationor deceleration demand), and then determines the wheel torque commandbased on the driver demand and powertrain limits. In addition, the VSC58 determines when and how much power or torque each power source needsto provide in order to meet the driver demand and/or vehicle demand andto achieve the operating points (torque and speed) of the engine 32 andM/G 34.

The VSC 58 selects the power and torque delivery mode based on thevehicle operating conditions and a predefined strategy. To this end, theVSC 58 receives a signal from a transmission range selector (PRND), anaccelerator pedal position sensor output (APPS), and a brake pedalposition sensor output (BPPS).

In alternative embodiments, the clutch 40 may be replaced with a torqueconverter unit including a torque converter and a lockup clutch orbypass clutch. The torque converter has torque multiplication effectswhen certain rotational speed differentials exist across the torqueconverter. During torque multiplication, the output torque of the torqueconverter is larger than that of the input torque due to torquemultiplication across the torque converter. Torque multiplication existsfor example, when the vehicle 30 is started from rest and the inputshaft to the torque converter begins to rotate, and the output shaftfrom the torque converter is still at rest or has just begun to rotate.

The lockup clutch or bypass clutch is used to lock out the torqueconverter such that the input and output torques for the downstreamtorque transfer device 40 are equal to one another, and the input andoutput rotational speeds for the device 40 are equal to one another. Alocked clutch eliminates slipping and driveline inefficiency across thetorque converter, for example, when the rotational speed ratio acrossthe torque converter is greater than approximately 0.8, and may increasefuel efficiency for the vehicle 30.

In other embodiments, the method described herein may be applied tohybrid vehicle having other system architectures. In one example, apowersplit vehicle architecture may be used. An example of a powersplithybrid vehicle is described in U.S. Pat. No. 6,994,360, issued on Feb.7, 2006, the contents of which are incorporated by reference in theirentirety herein.

For the vehicle 30, the engine 32 may be pulled up, or started for anumber of reasons. The engine 32 may be started when the vehicle powerdemand approaches or exceeds the available electrical power that may beprovided by the electric machine and the battery at that time. Forexample, the amount of electrical energy and power available may varybased on a battery state of charge, a maximum discharge rate of thebattery, the power, speed, and torque limits of the electric machine,and the like.

The vehicle power demand may include driver demand, such as from a tipin request, and other accessory vehicle loads, including cabinconditioning systems, exterior lights, and the like.

The engine 32 may be started when the vehicle torque demand approachesor exceeds the available electrical torque that may be provided by theelectric machine and the battery at that time. The available electricaltorque may depend on the battery state, and the torque limit of theelectric machine.

Other reasons for an engine pull up request include a battery state ofcharge reaching a minimum threshold value, a scheduled catalystaftertreatment regeneration process, and the like.

FIG. 3 illustrates a flowchart describing a method 100 for use incontrolling a hybrid vehicle according to an embodiment. In otherembodiments, the method 100 may include a greater or fewer number ofsteps, and various steps may be performed sequentially or in parallelwith one another. The steps in the method may also be ordereddifferently from the illustrated method in other embodiments.

The method 100 begins at 102 and proceeds to block 104 where itdetermines if the towing package for the vehicle is in use. The towingpackage may be determined to be in use based on an input to the controlsystem indicated that the electrical connector for the towing package isconnected, a load leveling system is activated, and/or sensors in thehitch receiver detect an attached load. If the towing package is not inuse, the method 100 returns to block 102. If the towing package is inuse at 104, the method 100 proceeds to block 106.

At block 106, the method 100 determines if the trailer weight isavailable. If the trailer weight is available, the method 100 proceedsto block 108 where it determines the weight. The weight may bedetermined using various methods as are known in the art. An example ofa method to determine trailer weight is described herein with referenceto FIG. 6. The trailer weight may also be determined based on a userinput, for example, through a user interface in the vehicle, where theuser inputs the weight of the trailer as determined by the user.

The method 100 then proceeds to block 110, where it determines if thetrailer weight is above a threshold value. For example, the thresholdvalue may be 3000 pounds, 5000 pounds, 10000 pounds, or any other value.The threshold value may be based on the vehicle electrical limits, thetowing capability of the vehicle, and other relevant factors.

If the trailer weight does not exceed the threshold value at 110, themethod 100 returns to the start 102 and operates the vehicle normally,or operates the vehicle as if no trailer was present.

If the trailer weight is above the threshold value at 110 or if thetrailer weight in unknown at 106, the method 100 proceeds to block 112.At 112, the method 100 causes the vehicle to be operated in a trailermode. The trailer mode may be set differently for different knowntrailer weights over the threshold value, and for different vehiclehybrid architectures.

In one embodiment, the method 100 causes the controller to command theengine to stop in response to vehicle power demand being less than acurrently available electrical power. The method 100 also causes thecontroller to command the engine to start while vehicle demand is lessthan the currently available electrical power to reduce a time delay forstarting the engine and increase a currently available vehicle power tomove the vehicle and trailer.

The engine may be started before an accelerator pedal is actuated. Forexample, if a vehicle is approaching a stop, such as a traffic light, orthe vehicle is placed into a park gear, the engine may be stopped toconserve fuel if the vehicle demand from accessory loads may be metelectronically. The engine may be restarted in response to a release ofa brake pedal or shifting the vehicle out of park, with the releasedetermined using the BPPS or the gear shift lever, in anticipation ofthe user engaging the accelerator pedal with a tip in request. Thisessentially disables electric creep for the vehicle, as the engine isstarted as the brake pedal is released and is therefore available tocause vehicle creep. The tip in request may exceed the electric machinecapability to accelerate the vehicle with a trailer attached, and so theengine is pulled up to meet a potential user request.

The engine may be stopped when the controller receives a braking requestfrom the BPPS, which would indicate that the driver demand is below roadload for the vehicle, for example, when driving downhill. The engine maybe pulled up or restarted as the brake pedal is released in anticipationof a tip in request at the accelerator pedal and the need for at leastthe engine to accelerate the vehicle.

The engine may be commanded to start by the controller when vehiclepower demand is at a first offset below the currently availableelectrical power. Vehicle power demand may include multiple powerdemands in the vehicle, including user requested power from a tip inrequest, and power demands from vehicle accessory loads. The currentlyavailable electrical power is the maximum power output from the electricmachine at that time, and may vary based on the battery state and theelectric machine operating state.

The engine may be pulled up for a vehicle without a load attached to thetowing package when the vehicle demand is at a second offset below thecurrently available electrical power. The second offset is less than thefirst offset such that there is a larger power delta between a scheduledengine pull up for a vehicle with a trailer than a vehicle without atrailer. In other words, the engine is pulled up sooner when the vehiclehas a load connected to the towing package based on the same currentlyavailable electrical power. The engine is pulled up sooner because thecurrently available electrical power may not be able to meet the samevehicle demand and accelerate the vehicle depending on the trailerweight. The engine is also pulled up sooner to meet user expectationssuch that when the user tips in, the engine is operating to providepower to accelerate the vehicle. The engine does not operate when thevehicle is towing and demand is below the first offset such that vehicledemand is met by the electric machine and user expectations of electriconly operation for a hybrid vehicle may be met, and to conserve fuel andincrease fuel economy for the vehicle.

The second offset may be implemented as a modification of the firstoffset. The second offset may be calculated or determined based on theweight of the trailer, if known. The first and/or the second offsets maybe predetermined and provided to the vehicle controller as a calibrationtable, or may be calculated in real time as the vehicle operates.

In another embodiment, the vehicle controller commands the engine tostart at a torque offset below an unloaded vehicle engine torque pull upschedule at 112. The controller may command the engine to start based onengine torque pull up schedule for an unloaded vehicle when the torquerequest for the engine reaches a specified value in the schedule. Theengine torque pull up schedule may be a calibration table referenced bythe controller. The torque offset may be based on the weight of the loadconnected to the towing package of the vehicle, such that a largertorque offset occurs for a heavier trailer load. In other embodiments,the torque offset may be a fixed value, for example, the offset may bebased on a maximum trailer weight for the vehicle.

The controller may command the engine to stop when the torque request isbelow the torque offset of the engine torque pull up schedule, therebyconserving fuel.

For the engine stop and start commands, there may be a filter used withthe pull up schedules to reduce hysteresis and the engine cycling on andoff based on a torque or power demand fluctuating near the offset valuefor pull up.

Of course, torque, power, and speed are related to one another. If twoof the three values are known, the third may be calculated. Power is theproduct of torque times rotational speed. Therefore, the engine power isthe engine torque times the engine output shaft speed. The electricmachine power is the electric machine torque output times the rotationalspeed of the electric machine output shaft. The power at the vehiclewheels is the torque at the wheels times the rotational speed of thewheels. Torque and power, as used in the examples herein, may beinterchanged using rotational speed as the simple conversion between thetwo.

At 112, the vehicle controller may also command a downstream clutch 40(or torque converter) to slip at vehicle launch. By slipping thedownstream clutch 40, vehicle power production may be increased as theengine speed is permitted to increase. Alternatively, vehicleacceleration may be increased by slipping the downstream clutch 40.

For a vehicle 30, the controller may disable a locked torque converter40 for a vehicle launch. By slipping the torque converter 40, theresulting torque multiplication across the torque converter results in agreater torque at the wheels to accelerate and propel the vehicle andtrailer.

From 112, the method 100 proceeds to block 114 and ends.

FIG. 4 illustrates a timing chart for a hybrid vehicle according to thepresent disclosure. The hybrid vehicle has a load attached to the towingpackage such that the vehicle is trailering. The controller caused thevehicle to enter the trailer mode based on the trailer weight. Thevehicle is at rest in zone 150. The user has the brake pedal depressedas shown by the brake input 152. The electric machine, as shown by theelectric machine speed 154, is at rest. The engine, as shown by theengine speed 156, is also at rest. At time 158, the user begins torelease the brake pedal 152. The controller then requests an engine pullup, as shown by engine command 160.

In zone 162, the engine speed 156 increases as the engine is pulled up.At time 164, the engine is operating at idle speed, and the brake pedaland accelerator pedal are not engaged. The vehicle may be creeping atthis point.

At time 166, the user provides a tip in request 168 to the acceleratorpedal. The engine speed and electric machine speed (and torque andpower) increase to meet the user and vehicle demand. By starting theengine as the brake pedal is released, the engine is operational as soonas the accelerator pedal is engaged and there is no delay in vehicleacceleration and response due to an engine start sequence.

FIG. 5 illustrates a chart for engine pull up based on vehicle power fora hybrid vehicle according to an embodiment. The requested or demandedvehicle power is shown at 180. The maximum available electrical power isillustrated at 182. The maximum available electrical power may bedetermined using the battery state of charge, the vehicle speed, theoperating state of the electric machine, and other vehicle inputs.Multiple charts or tables may provide maximum electrical power andvarying trailer loads.

The engine may be scheduled to be pulled up at 184, when the vehicledemand reaches a threshold 186 that corresponds to an engine pull up fora vehicle with no load connected to the towing package, or a load belowthe weight threshold. Threshold 186 is at an offset 188 below themaximum available electrical power 182 to provide a reserve or bufferregion.

When the hybrid vehicle has a load attached to the towing package suchthat the vehicle is trailering, the controller caused the vehicle toenter the trailer mode based on the trailer weight. In the trailer mode,the controller commands an engine pull up at 190 when the vehicle demandreaches a threshold 192 that corresponds to an engine pull up for avehicle with a load connected to the towing package, which may be a loadabove the weight threshold. Threshold 192 is at an offset 194 below thethreshold 186 to provide an additional reserve or buffer region suchthat the engine is operating when a vehicle power demand is received toprovide the requested power. The offset 194 may vary based on the weightof the trailer, and may increase in value as the weight increases.

Note that although the maximum available electrical energy may be thesame between a vehicle that is trailering and one that is unladen, alarger portion of the electrical energy may be used with the traileringvehicle to meet the same vehicle demand. For example, based on thevehicle state and maximum available electrical energy, an unladenvehicle may be able to operate electrically up to 75% wide openthrottle. The same vehicle having the same vehicle states, but with anattached trailer, may only be able to operate electrically up to 60%wide open throttle.

A similar schematic as FIG. 5 may be provided with torque replacingpower, and having an offset between torque for engine pull up for anunladen vehicle and a vehicle towing a trailer. The engine pull uptorque threshold for a vehicle towing a trailer is less than the enginepull up torque threshold for an unladen vehicle. The offset between thepull up torque thresholds may be based on the weight of the trailer,such that the offset increases as the trailer weight increases.

FIG. 6 is a flowchart of a method 200 to determine a trailer weight foruse with the method 100 shown in FIG. 3 according to an embodiment. Themethod 200 may be used at step 108 in method 100 for example. The method200 starts at 202, and proceeds to block 204 where the controllerreceives the actual grade based acceleration of the vehicle from theroad condition monitoring accelerometer, or the like.

The method 200 then calculates the change in acceleration (Aaccel) at206. The grade based acceleration is added to an expected level groundacceleration, or a zero grade acceleration and this total is thensubtracted from the actual vehicle acceleration to provide the change inacceleration.

The method 200 calculates the change in mass (Amass) at 208. The changeis mass is calculated using Newton's second law, or Δmass=F/Δaccel, andassuming that any change in acceleration is due to the change in mass.The force, F, that is used is based on the torque at the wheels and theradius of the wheel assembly. The mass is scaled by a percent differencein the acceleration, and the base mass is then subtracted to calculateΔmass.

At 210, the method 200 determines if a trailer is attached, using inputfrom block 104 in method 100, for example. If a trailer is attached, themethod 200 proceeds to block 212 and the controller sets the trailerweight as equal to Amass. If there is no trailer attached, the method200 proceeds to block 214 and the controller sets the vehicle payload asequal to Amass. The method then goes to block 216 from either 212 or 214and ends.

As such, various embodiments according to the present disclosure providefor increased fuel efficiency and user expectations of a hybrid vehicletowing a load or a trailer. The vehicle may be operated in an electriconly mode with the electric machine propelling the vehicle and theengine off even when the vehicle is towing. When the vehicle is towing,especially for a heavier load such as a 5000 pound, 10,000 pound, orheavier trailer, the electric machine may have a reduced ability toaccelerate and propel the vehicle and trailer due to the added weight ofthe trailer. Instead of disabling an electric only mode, as has beendone in the prior art, the present disclosure allows the vehicle tooperate electrically depending on the vehicle demand. The presentdisclosure provides a vehicle where the engine may be shut down whilethe vehicle is operating and vehicle demand may be met electrically inorder to conserve fuel and meet user expectations. The engine may bepulled up in advance of a typical vehicle pull up schedule in order toanticipate and meet user demand and vehicle demand with the added weightof a trailer.

While exemplary embodiments are described above, it is not intended thatthese embodiments describe all possible forms of the invention. Rather,the words used in the specification are words of description rather thanlimitation, and it is understood that various changes may be madewithout departing from the spirit and scope of the invention.Additionally, the features of various implementing embodiments may becombined to form further embodiments of the invention.

What is claimed is:
 1. A method for controlling a hybrid vehicle havinga trailer hitch comprising: detecting the hitch being in use; commandingan engine to stop to increase fuel efficiency; and in response to atorque to accelerate the vehicle being within a predetermined offsetbelow an engine torque pull up schedule for an unladen vehicle,commanding the engine to start to reduce a time delay for starting theengine and increase vehicle torque to move the vehicle.
 2. The method ofclaim 1 further comprising calculating a weight of a load connected tothe hitch; wherein the predetermined offset is based on the weight ofthe load.
 3. The method of claim 2 wherein the predetermined offsetincreases with increasing trailer weight.
 4. The method of claim 2further comprising inputting the weight into a calibration table todetermine the predetermined offset.
 5. The method of claim 2 furthercomprising calculating a weight of a load connected to the hitch;wherein the torque to accelerate the vehicle is based on the weight ofthe load.
 6. The method of claim 5 further comprising measuring anactual vehicle acceleration; wherein the weight of the load iscalculated based on the actual vehicle acceleration compared to a gradebased vehicle acceleration.
 7. The method of claim 1 further comprisingreceiving a user input indicating a weight of a load connected to thehitch; wherein the predetermined offset is based on the weight of theload.
 8. The method of claim 1 wherein detecting the hitch being in useis based on receiving a signal from a trailer tow module.
 9. The methodof claim 1 wherein detecting the hitch being in use is based onreceiving a signal from a manual switch.
 10. The method of claim 1further comprising comparing a weight of a load connected to the hitchto a threshold value; and setting the predetermined offset to zero ifthe weight is less than the threshold value.
 11. The method of claim 1wherein the engine is commanded to stop to increase fuel efficiency whena vehicle torque request is below the offset of the engine torque pullup schedule.
 12. The method of claim 1 wherein the engine is commandedto start in response to the torque to accelerate the vehicle beingwithin the predetermined offset below the engine torque pull up schedulefor the unladen vehicle to reduce the time delay for starting the engineand increase vehicle torque to move the vehicle at a vehicle launchcondition while slipping a downstream clutch, the downstream clutchconnecting an electric machine to a transmission gearbox.
 13. The methodof claim 12 wherein the downstream clutch is a lockout clutch for atorque converter, and slipping the downstream clutch at a vehicle launchcondition causes torque multiplication across the torque converter. 14.The method of claim 12 wherein a vehicle speed is substantially zeroprior to the vehicle launch condition.
 15. The method of claim 1 whereinthe engine torque pull up schedule for an unladen vehicle is a functionof an available vehicle electrical torque.
 16. The method of claim 15wherein the available vehicle electrical torque is a function of atraction battery state and a torque limit of an electric machine.